Determination of Phase Transition Temperatures for Atomistic Models of Lipids from Temperature-Dependent Stripe Domain Growth Kinetics

Abstract

Many lipid bilayers undergo a reversible order-disorder transition between the gel and liquid crystalline (LC) phases at a main phase transition temperature T(m) that is an important characteristic property of the lipid. Although T(m) should serve as a useful standard for validation and calibration of simulation models of lipid bilayers, its evaluation within simulations is difficult due to the slow kinetics of the gel-LC transition, especially near T(m). A stripe growth strategy for calculating T(m), which aims to bypass the slowest steps in this transition, has been applied to dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine bilayers represented with a commonly used united-atom force field. The strategy consists of initial preparation of a bilayer containing gel and LC domains arranged as parallel stripes, observation of the direction and rate of domain growth over a range of temperatures, and fitting rates to an Arrhenius-like equation for their temperature dependence that crosses zero at T(m). Calculated T(m)'s for both lipids are 5-6 degrees lower than their experimental values, in much closer agreement with experiment than suggested by recent simulations that simulate heating and cooling of bilayer patches. The stripe growth method also yields rates of phase front propagation that are in order-of-magnitude agreement with experimental estimates, as well as insight into glycerol backbone disordering at the LC-gel interface.